Soluble polymeric additives have been known to reduce the drag, under turbulent flow conditions, and there is an extensive literature which treats this subject. For example, very high molecular weight additives such as polyethylene oxides, polyacrylamides, carboxymethycellulose and guar gum may result in high levels of drag reduction even at very low polymer concentrations. However these additives degrade very rapidly, due to the high stress levels to which they are subjected in the turbulent field, and hence are more useful in "one shot" applications, such as flow through a fire hose, than in flow through lengthy pipelines or other conducits. It has been determined that the mechanisms responsible for drag reduction using polymeric additives are located physically in or near the quasilaminar sublayer immediately adjacent to the solid surface. Although the reductions in drag obtainable using polymeric additives may be quite large the additives have not been extensively used: in addition to the problem of rapid degredation rate one notes that they are expensive and are less efficient in large scale systems than they are in a laboratory apparatus having small dimensions. Finally they are not of value as process fluids (as in a heat transfer loop in the chemical process industries) because, although the pumping power requirements are reduced below the levels exhibited by the unmodified carrier liquid, the heat transfer rates are reduced even more greatly. That is to say, the process fluid's efficiency, defined as the rate of heat transfer per unit of power expended, is lower for the drag-reducing solution than for the unmodified liquid. A comprehensive review of this entire subject of turbulent drag reduction has recently been provided by J. W. Hoyt in his article entitled "The Effect of Additives on Fluid Friction" Trans A.S.M.E. (J. Basic Eng.) 94D, 258 (1972). He shows that the maximum reduction in drag observed to date, with polymeric additives, is in the neighborhood of 80%.
Separately, it has been known for a long time that small particles, such as dusts, sediments and paper pulp fibers, when suspended in a turbulent fluid, will also reduce the drag on an object or the gradient of the fluid head in conduits. The magnitude of the drag reduction obtainable using suspended solids has not been as dramatic as in the case of polymeric additives, and the mechanisms by means of which this drag reduction occurs with suspended solids have not been elucidated. Recently, J. W. Hoyt in his article "Turbulent Flow of Drag-Reducing Suspensions" Naval Undersea Center Report TP 299, San Diego (1972) has been able to show that the reduction in drag obtainable with some fibrous additives, e.g. glass fibers and asbestos particles, may be quite large and possibly worth exploiting.
R. C. Vaseleski has been able to show in his M.Ch.E. Thesis at the University of Delaware, May, 1973, that the use of fibrous additives affects the turbulence in the "turbulent core" in flow through a conduit; i.e., the region removed from the laminar sublayer adjacent to the wall. Correspondingly, in the case of flow around submerged objects these fibrous additives would be expected to influence the turbulent field at some distance from the submerged object. A most important result of these observations is that the observable drag reduction is not affected adversely by increasing the scale of the system, at a constant value of the Reynolds number.